How many people know of your interest in this?

Your father would probably never even know of Derek's interest.

I know of the hon. Gentleman's interest in the subject.

I know of his close interest in that matter.

This was just a courtesy call to let you know of our interest.

"I know not of my father's interest in any pizza from any place on 118th Street."

Now he needs to let them know of his interest.

He couldn't fathom why she was so anxious that no one else should know of his interest in her.

It does no harm to let them know of your interest while a course is in preparation.

By that time, you see, I knew of his interest in the human brain.